apps/enc.c: avoid signed integer overflow on bufsize assignment

The calculated option value, while being long-typed, is not checked
for fitting into int-sized bufsize.  Avoid overflow by throwing error
if it is bigger than INT_MAX and document that behaviour.

Fixes: 7e1b748570 "Big apps cleanup (option-parsing, etc)"
Resolves: https://scan5.scan.coverity.com/#/project-view/65248/10222?selectedIssue=1665149
References: https://github.com/openssl/project/issues/1362
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28405)

apps/enc.c

@@ -260,6 +260,8 @@ int enc_main(int argc, char **argv)
                 goto opthelp;
             if (k)
                 n *= 1024;
+            if (n > INT_MAX)
+                goto opthelp;
             bsize = (int)n;
             break;
         case OPT_K:

doc/man1/openssl-enc.pod.in

@@ -196,6 +196,7 @@ or decryption.
 =item B<-bufsize> I<number>
 
 Set the buffer size for I/O.
+The maximum size that can be specified is B<2^31-1> (2147483647) bytes.
 
 =item B<-nopad>